The several components are widely used as plant nutrients or soil additives for modifying soil acidity or converting nutrients present in the soil to available forms. Similarly, urea and ammonium nitrate particles are commercially available and are generally applied to the soil as such. However, it is often desirable to apply several additives, such as the major or minor nutrients, simultaneously. This can be accomplished with a physical mixture of powders or particles of the several components. One disadvantage of this approach is the practical impossibility of obtaining a mixture of sufficiently uniform particle size to prevent settling and segregation of the several components during transportation, storage or use. This approach also does not take advantage of at least one factor governing the availability of plant nutrients to crops. Many soils are generally basic in character, having pH levels above about 8.0, while the plant nutrients may be relatively immobile, i.e., water insoluble, under basic conditions.
The mobility of these nutrients and, consequently, their availability to the crops, can be increased by reducing soil pH. This is accomplished to some extent by the action of nitrifying bacteria on urea or ammonium nitrate by which all the nitrogen is converted to available nitrates. However, this effect is apparent primarily, if not exclusively, in the immediate vicinity of the nitrifier particles. Consequently, broadcasting a physical mixture would not place the basic, immobile additives such as magnesium oxide, iron sulfate, manganese sulfate, etc., in close enough proximity to the nitrifier particles to take the greatest advantage of the pH reduction occurring in the immediate particle vicinity. Increased mobility associated with acidification occurs whether the additional soil additives or nutrients are added in soluble or mobile forms. Soluble materials might otherwise be converted to the insoluble, basic forms in a basic environment.
One obvious solution is to combine the additive with the nitrifier in the form of a particle during the production of the nitrifier particle. Presumably, this could be done by mixing the additive with the melt or solution during the prilling operation. This is often impossible, however, due to the compatibility of some soil additives, e.g., elemental sulfur, and molten urea or ammonium nitrate. Secondly, it introduces complexities into the prilling operation which may or may not be easily solvable or economically acceptable. Prilling procedures usually involve large scale, continuous operations which would have to be interrupted periodically to produce custom formulations of different compositions as the demand required.
Due to the wide commercial availability of nitrifier particles as such, it would be most desirable to devise a method for combining the additional nutrients and with the particles on a small scale without appreciable difficulty; a method that would not interfere with the large scale particle production in the first instance. However, this approach is also complicated by several factors, primarily the physical structure of the nitrifier particles and their incompatibility with other soil amendments.
For instance, prills have low porosity; essentially no surface porosity for practical purposes. Thus very little if any material can be added to the particle surface by a simple procedure. The secondary components cannot be added practically in aqueous solutions since the nitrifiers are hygroscopic, adsorb water very rapidly, and dissolve or become sticky and cohesive at very low water levels. Impregnation instantly results in an unhandleable cemented mass.
It thus remains to attempt the combination of nitrifier particles and additional amendments by some procedure that does not require the use of sufficient free water to dissolve those components. The majority of the additives do not fuse at temperatures below the nitrifier melting point. Even sulfur, which does have a relatively low melting point, is not compatible with the nitrifiers even in the molten form.
These difficulties have not completely eliminated all efforts to produce such materials. One suggestion involves coating urea particles with wax and then introducing the amendments which adhere to the wax coating. The prior art accurately observes, however, that in order to produce a coating which is sufficiently cohesive to retain the additional components, one also produces particles which agglomerate, thereby introducing obvious complications and disadvantages. The prior art also suggests that agglomeration can be overcome, at least to some extent, by adding clays such as keiselguhr to the combination of urea, wax and sulfur or other additives. While to some extent accomplishing the objective, this approach also has several drawbacks. It requires the addition of materials such as waxes and clays which have little or no utility in the final product (except for water-proofing if desired) resulting in increased manufacturing cost and expense of shipping, handling and application.
It is therefore an object of this invention to provide an improved, particulate multicomponent soil amendment containing urea or ammonium nitrate, calcium sulfate and at least one additional soil additive or plant nutrient. Another object is the provision of a simple procedure for producing such multicomponent soil amendments. Another object involves the provision of a method for producing intimate mixtures of soil additives and/or plant nutrients from readily available materials such as commercial prills. Another object is the provision of an improved method for applying agronimically effective amounts of soil amendments and/or plant nutrients to the soil. A preferred objective is the provision of multicomponent soil amendments containing urea, calcium sulfate and additional soil additives in which the additional additives are in such close proximity to the urea that maximum benefit is derived from the acidic environment created in the immediate proximity of the urea particles by nitrification.